The present invention relates generally to electroslag processing of titanium base alloys to achieve low nitrogen concentrations. More specifically, it relates to carrying out the electroslag refining of titanium base alloys so as to reduce the concentration of nitrogen below that which is conventionally present.
It is known that the processing relatively large bodies of metal, such as superalloys and titanium alloys, is accompanied by many problems which derive from the bulky volume of the body of metal itself. Such processing involves problems of sequential heating and forming and cooling and reheating of the large bodies of the order of 5,000 to 35,000 pounds or more in order to control grain size, other microstructure and other properties. Such problems also involve segregation of the ingredients of alloys in large metal bodies as processing by melting and similar operations is carried out. A sequence of processing operations is sometimes selected in order to overcome the difficulties which arise through the use of bulk processing and refining operations.
One such sequence of steps involves a sequence of vacuum induction melting followed by electroslag refining and followed, in turn, by vacuum arc refining and followed, again in turn, by mechanical working through forging and drawing types of operations. While the metal produced by such a sequence of steps is highly useful and the metal product itself is quite valuable, the processing through the several steps is expensive and time-consuming.
For example, the vacuum induction melting of scrap metal into a large body of metal of 20,000 to 35,000 pounds or more can be very useful in recovery of the scrap material. The scrap may be combined with virgin metal to achieve a nominal alloy composition desired and also to render the processing economically sound. The size range is important for scrap remelting economics. According to this process, the scrap and other metal is processed through the vacuum induction melting steps so that a large ingot is formed and this ingot has considerably more value than the scrap and other material used in forming the ingot. Following this conventional processing, the large ingot product is usually found to contain one or more of three types of defects and specifically voids, slag inclusions and macrosegregation.
This recovery of scrap into an ingot is the first step in a refining process which involves several sequential processing steps. Some of these steps are included in the subsequent processing specifically to cure the defects generated during the prior processing. For example, such a large ingot may then be processed through an electroslag refining step to remove a significant portion of the oxide and sulfide which may be present in the ingot as a result of the ingot being formed at least in part from scrap material.
Electroslag refining is a well-known process which has been used industrially for a number of years. Such a process is described, for example, on pages 82-84 of a text on metal processing entitled "Superalloys, Supercomposites, and Superceramics". This book is edited by John K. Tien and Thomas Caulfield and is published by Academic Press, Inc. of Harcourt Brace Jovanovich, and bears the copyright of 1989. The use of this electroslag refining process is responsible for removal of oxide, sulfide and other impurities from the vacuum induction melted ingot so that the product of the processing has lower concentrations of these impurities. The product of the electroslag refining is also largely free of voids and slag inclusions.
However, a problem arises in the electroslag refining process because of the formation of a relatively deep melt pool as the process is carried out. The deep melt pool results in a degree of ingredient macrosegregation and in a less desirable microstructure. Defects produced by macrosegregation are visually apparent and are called "freckles". One way to reduce freckles is by reducing the diameter of the formed ingot but such reduction can also adversely affect economics of the processing.
To overcome this deep melt pool problem, a subsequent processing operation is employed in combination with the electroslag refining, particularly to reduce the depth of the melt pool and the segregation and microstructure problems which result from the deeper pool. This latter processing is a vacuum arc refining and it is also carried out by a conventional and well-known processing technique.
The vacuum arc refining starts with the ingot produced by the electroslag refining and processes the metal through the vacuum arc steps to produce a relatively shallow melt pool and to produce better microstructure, and possibly a lower nitrogen content, as a result. Again, for reasons of economic processing, a relatively large ingot of the order of 10 to 40 tons is processed through the electroslag refining and then through the vacuum arc refining. However, the large ingots of this processing has a large grain size and may contain defects called "dirty" white spots.
Following the vacuum arc refining, the ingot of this processing is then mechanically worked to yield a metal stock which has better microstructure. Such a mechanical working may, for example, involve a combination of steps of forging and drawing to lead to a relatively smaller grain size. The thermomechanical processing of such a large ingot requires a large space on a factory floor and requires large and expensive equipment as well as large and costly energy input.
The conventional processing as described immediately above has been found necessary over a period of time in order to achieve the very desirable microstructure in the metal product of the processing. As is indicated above in describing the background of this art, one of the problems is that a first processing step results in some deficiency in the product of that step so that another, and second processing step is combined with the first in order to overcome the deficiency of the initial or earlier step in the processing. However, when the necessary combination of steps is employed, a successful and beneficial product with a desirable microstructure is produced. The drawback of the use of this recited combination of processing steps is that very extensive and expensive equipment is needed in order to carry out the sequence of processing steps and further a great deal of processing time and heating and cooling energy is employed in order to carry out each of the processing steps and to go from one step to the next step of the sequence as set forth above.
The processing as described above has been employed in the application of superalloys such as IN-718 and Rene 95. For some alloys the sequence of steps has led to successful production of alloy billets, the composition and crystal structure of which are within specifications so that the alloys can be used as produced. For other superalloys, and specifically for the Rene 95 alloy, it is usual for metal processors to complete the sequence of operations leading to specification material by adding the processing of large ingot products of the processing through powder metallurgy techniques. Where such powder metallurgical techniques were employed, the first steps in completing the sequence are the melting of the large alloy ingot and gas atomization of the melt by conventional remotely coupled atomization techniques. This is followed by screening the powder which is produced by the atomization. The selected fraction of the screened powder is then conventionally enclosed within a can of soft steel, for example, and the can is HIPed to consolidate the powder into a useful form. Such HIPing may be followed by extruding or other conventional processing steps to bring the consolidated product to a useable form.
An alternative to the powder metallurgy processing as described immediately above is an alternative conventional process known as spray forming. Spray forming has been described in a number of patents including the U.S. Pat. Nos. 3,909,921; 3,826,301; 4,926,923; 4,779,802; 5,004,153; as well as a number of other such patents.
In general, the spray forming process has been gaining additional industrial use as improvements have been made in such processing, particularly because it involves fewer steps and has a cost advantage over conventional powder metallurgy techniques so there is a tendency toward the use of the spray forming process where it yields products which are comparable and competitive with the products of the conventional powder metallurgy processing.
It has been recognized that in the processing of titanium base alloys a great deal of technology has been developed over a period of time in the electroslag refining of the titanium base alloys. Among the literature references which relate to the electroslag refining of titanium based alloys is the following:
(1) OV Tarlov, AP Maksimov, VI Padchenko, "About the Oxygen Behaviour in Titanium Electroslag Remelting", Donetsk Polytechnical Institute, Advances in Special Electrometalurgy (USSR) 7, (2), 95-98 Apr.-Jun. 1991.
(2) A. Mitchell, "The Production of High-Quality Materials by Special Melting Processes", J. Vac. Sci. Technol. A 5, (4-IV), 2672-2677 Jul.-Aug. 1987.
(3) H. Jaeger, R. Tarmann, R. Froehlich, J. Baumgartner, "New Production Routes for Vacuum Melted Aerospace Materials", Iron and Steel Institute of Japan, Keidanren Kaikan, Otemachi 1-9-4, Chiyodaku, Tokoyo 100, Japan, 1982 .
(4) EL Morosov, AD Tchutchurukin, "Electroslag Remelting of Titanium Ingots", Plenum Press, 233 Spring St., New York, N.Y. 10013, 1982, 161-167.
(5) EI Morozov, MI Musatov, AD Churchuryukin, Sh Fridman, "Investigation of Various Methods of Melting and Casting of Titanium Alloys", TMS/AIME, P.O. Box 430, 420 Commonwealth Dr., Warrendale, Pa., 15086, 1980 .
(6) RA Beall, PG Clites, "Large-Scale Electroslag Melting of Ti", The Electroslag Melting Process Bull. 669, U.S. Bureau of Mines, 1976, 97-108.
(7) VZ Kutsova, DE Belokurov, "Reprocessing Titanium Production Wastes by Electroslag Remelting with Nonexpandable Electrode", Liteinoe Proizvodstov n Apr. 4, 1991 p. 18-19, 1991.
(8) HB Bornberger, FH Froes, "Melting of Titanium", Journal of Metals v 36 n 12 Cec. 1984 p. 39-47.
(9) RA Beall, PG Clites, JT Dunham, RH Nafziger, "Titanium Melting by the Electroslag Process Final Summary Report, Jun. 1965-Sep. 1968 (Titanium Melting by Electroslag Process)", Report No.: AD-697723; USBM-RC-1351.
(10) CE Armantrout, RA Beall, JT Dunham, "Properties of Wrought Shapes Formed from Electroslag-Melted Titanium", Metallurgical Society of AIME, and American Society for Metals, International Conference on Titanium, London, England, May 21-24, 1968, Proceedings. P. 67-74 ./A70-34351 17-17/.
(11) VN Radchenko, OV Tarlov, AP Maksimov, "Oxygen Behavior in Electroslag Remelting of Titanium", Probl. Spets. Elektrometall., 1991.
(12) VZ Kutsova, DE Belokurov, "Processing of Titanium Industry Wastes by Electroslag Remelting with Nonconsumable Electrode", Journal: Liteinoe Proizvod., 1991 pp. 18-19.
(13) VN Zamkov, TM Shpak, NG Zaitseva, Yu.K Novikov, "Electroslag Remelting of Complex Titanium Alloys", Probl. Metallurg. Pr-va, Kiev, 1990, pp. 87-9.
(14) Vaclav Klabik, Vaclav Landa, Miroslav Cadil, "Ingot Manufacture from Superconductive Niobium-Titanium Alloy", Czechoslovakia; CS 252053 B1, Date: 880625.
(15) Toshio Onoe, Tatsuhiko Sodo, Seiji Nishi, "Fluxes for Electroslag Remelting", JP 86288025 A2; JP 61288025, Date: 861218.
(16) "Electroslag Remelting of Titanium - in Protective Atmosphere38 U.S. Pat. No. 3,989,091, issued Nov. 2, 1976. The U.S. Pat. No. 3,989,091 discloses what appears to be the use of an inert gas in connection with an apparent continuous casting of a titanium ingot coupled with electroslag remelting within a "cooled mould".
(17) VE Roshchin, D Ya Povolotskii, PP Biryukov, "Behavior of Nitrogen and Nitride Inclusions in Electroslag Remelting of Titanium-Bearing Steel", Steel USSR 10, (2), 80-81 Feb. 1980.
In all of the literature concerning the electroslag refining of titanium based alloys there is no description of the effect of nitrogen on the properties of the titanium based alloys. I have found that it is highly desirable to reduce the concentration of nitrogen in the titanium based alloys so as to enhance the properties of the titanium based alloys. In particular, I have found that it is desirable to reduce the nitrogen content of alloys of titanium which are prepared by electroslag refining.